


Pines & Portals

by Salacious_Sovereign



Category: Gravity Falls
Genre: Anal Sex, Bisexuality, F/F, F/M, Lolicon, Orgasm Delay/Denial, Portals, Semi-Public Sex, Sibling Incest
Language: English
Status: Completed
Published: 2021-02-24
Updated: 2021-02-24
Packaged: 2021-03-18 16:41:49
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 4,751
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29371758
Author URL: https://archiveofourown.org/users/Salacious_Sovereign/pseuds/Salacious_Sovereign
Summary: Just when boredom strikes in their middle school math class, a previous experiment with magic gives Mabel's day just the spice it needed to be enjoyable.
Relationships: Dipper Pines/Mabel Pines, Mabel Pines/Original Female Character(s)
Comments: 3
Kudos: 28





	Pines & Portals

**Author's Note:**

> Disclaimer: I do not condone any sexual activity with or towards minors. It is a crime almost everywhere worldwide for a reason. Children can not consent, as they have not developed enough nor been taught enough to do so. Keep it fictional.

Mabel Pines was **_bored_** , emphasis absolutely necessary.

She was currently in math class, listening to a lecture about some X's or Y's or whatever, desperately wishing for something to happen. She plopped her head in her hand, groaning a bit and attracting the attention of her friend sitting next to her at their desk. 

Stacy Fortwright, a girl who was definitely pretty at least, with curly shoulder-length black hair, and bright hazel eyes, she could turn some heads for sure. She and Mabel had been friends for quite some time, ever since she'd moved out to California, and the overexcited sweater-dweller had gotten to her by use of her in-your-face attitude and general kindness. She was her closest confidant and friend in California, outside of Dipper of course, and the two had gotten along swimmingly. She knew all of Mabel's secrets, even some Dipper doesn't know (girl things), and she's been pretty loyal to Mabel and her brother.

So of course Stacy knew that her and Dipper were fucking.

Yes, her and her brother had been in a highly sexual relationship for a while. On the bus ride home from that first fateful summer at Gravity Falls, she'd gotten bored and noticed that he'd had a bit of a boner, and she'd indulged in her curiosity with it. One thing turns to another and she's leaning over and licking his penis up and down, bro-cum splattering all over her face. That'd been one of the best moments of her life.

Ever since then her and Dipper would make time out of their day to have sex, a handie on the bus here, a fuck in the bushes there, some oral in the bathroom everywhere. She loved that they'd become so much closer from their constant fucking, and the pleasure she was having at Dipper's improving skill was nothing to scoff at too.

Mabel of course, had decided that it'd be fun to do more however, and pushed for more interesting things to add some more spice. Not that she was getting bored, heaven forbid, but she was always wanting to try something new with her Dipping-Dot. She'd loved all the kinky things they'd get up to, from her eating heaps of his cum on her lunch, savoring the gooey and salty perversion of her meal. She's worn vibrators under her sweater, the sensation on her nipples all day leaving her soaked and mewling by the end. She's even poked a couple fingers up her brother's butt when sucking his juicy (well, she thought so anyways) every now and then, something he proved surprisingly accepting of once she'd actually done it.

However, since they'd been very involved in the creatures of Gravity Falls, they also introduced magic into their sex lives. Things like having Dipper increase the size of his penis with the crystal flashlight, something that made her butt had feel better than ever... or more sore than ever. They'd switched bodies and had sex with each other, _as_ each other, and she found that while having a dick was fun, but she preferred getting a dick overall. They'd even used the copying machine to have gangbangs made entirely out of themselves! There was _so much Dipper_ , and so little her, and she'd loved every second of being swarmed by her brother's cock.

That was what they were doing today in fact, and she was reminded of that by the sensation of a finger tracing it's way around her labia, making her gasp lightly and swiftly grip her desk. She looked over toward Dipper a couple rows ahead of her, and sure enough, after a few moments of searching she saw the outline of his hand making slow circles around something in his pocket, and as she could feel the corresponding movements on her lower lips, she realized what was happening to her.

This was her suggestion actually. Recently, they'd found a way to actually make teleportation portals, and were both super excited about doing so. Unfortunately, unless they had ridiculous magical powers, they couldn't make the portals very big. However, Mabel had been super horny around that time, and had a stroke of pure genius: to try and link a portal directly to her holes, to let Dipper use them whenever and wherever he wanted, regardless of time or place. He'd initially been hesitant, as he was worried about someone finding them, but she'd shot him down, filled with lust at the thought of being played with at any time unexpectedly by her brotherly lover, and gave a solid argument that even if the portal was found, no one would have suspected that it lead to a real girl, especially pinning down her specifically. He'd agreed, and they'd gotten to work on it. Well, mostly Dipper, as he was the one with the magical know-how, but she did act as like, a magical-lab-assistant, as she desperately wanted their idea to come to fruition.

And so it eventually did, as after a month of working and researching on this with their freetime (not all of it, as they were both much too horny to NOT fuck at least a little, especially whenever they were working on something that revolved around sex), and came out with a working product. The end result ended up being a hand mirror without the handle, and instead of showing a face when looked into, it showed and allowed touching of Mabel's pussy, ass, and even mouth, really anywhere her bro liked to stick his penis into. And as an added bonus, Dipper could switch it about using his own magical skill. She'd been so excited to test it, but told him to wait on using it, as she wanted to be taken completely off-guard when he finally used it.

And so, weeks had passed by, and they'd lived their lives and had their fun, all the while Mabel had entirely forgotten about the mirror, being more focused on the currently active parts of her life. But apparently Dipper hadn't, he'd just been biding his time for whenever his sister was one-hundred percent unaware, and set to strike when that time came. Thankfully, he'd started small, with slow, almost glacial loops around the outside of her pussylips. Just a single little finger, going as slow as humanly possible, and setting a fire in her honeypot like never before.

Mabel finally knew exactly what was happening to her, and she quickly clenched her teeth to not let any of her soft moans escape into the classroom, as she knew she wasn't a quiet lover and she didn't need to get caught. Though Dipper was making that task very difficult, with his soft tracing of her pussy, and the knowledge that she was right here in class, and she could just moan loudly and let the whole room, maybe even the whole _school_ , know that her brother was playing with her pussy and she was loving it was tempting on a sexual level, but logically she was also terrified and embarrassed of that, adding to her mess of emotions at the moment.

She groaned as after a good half-minute, her brother had finally made a single loop around her lips, and she was torn between loving him for his gentle caress, and hating him for being so damn _slow_ about it. Oh, the situation was agony to her, and she could feel her panties begin to moisten around her privates, the whole scenario making her way hornier much faster than if they were alone. She gave off small gasps, soft enough to not let the class know what she was enduring.

As much as this slow treatment was tearing her apart inside, she was just as much reveling it. She knew she was a slut for being risky in their sex, as this wasn't the only time where her and Dipper had had their fun where they could easily be caught. The difference here though was the complete lack of preparation she had for today, as well as just how _easy_ it would be to get caught. This wasn't giving a blowjob behind a pool shed, or fucking deep in the woods, this was right here in school, surrounded on all sides by her friends and classmates, and just a single slip could have them on the path to discovering how much of a depraved pervert she was.

And it was **_hot_**. 

Her breath hitched as Dipper suddenly sped up his movements, making a fast three loops around her in just a couple of seconds, and she had to quickly bite her sleeve to not cry out in the class. This was enough movement for Stacy to notice her, and she felt her friend's hand on her back as she leaned in, concernedly asking "Mabel, are you okay?"

Mabel let go of her grip on her sleeve, and gave a soft, shuddering response of "D-Dipper... Portal..." and moved to bite her sleeve again to endure her brother's ministrations.

Stacy took a second to process what was said, and then got a vicious grin as she comprehended Mabel's words, and leaned close to her, stopping right by her ear, and began to whisper in a hushed, sensual, and mocking tone. "Oh, is your brother playing with your tight little pussy? Right here in class?" she asked, and Mabel lightly shuddered, her friend not helping her situation of not wanting to get caught.

Stacy was very aware of what Mabel liked, and right now Mabel she wasn't sure if she wanted to tear her dumb nose off... or kiss her until they couldn't breathe.

"Well, I asked you a question, _slut_." Stacy said, putting emphasis on the last word, and Mabel nodded, feeling way too horny to formulate any words. "That's what I thought. So, trying not to get caught? To not let all of these friends you've made know you love getting your little cunt stroked right next to them? Why would you do that, you know you're a slut after all. You know you _love_ that so much, so why lie? Just let them know. Let them know how much of a dirty slut you are."

Mabel bit harder into her sleeve as her friend continued whispering these little depravities into her ear, knowing how hot and bothered it got Mabel. "And what if they found out that of all people, it was your brother? How would they react if they knew that Mabel was an incestuous little freak, who could only get off if her blood twin was the one fucking her tight cunt, hmm?"

Stacy geared up to continue belittling her friend when she saw her friend's hand grip the desk tightly, hard enough for her to visibly see the strain, and she paused in her verbal assault. In a much kinder tone, she asked "Hey Mabel, what's happening now, can you handle it?" Because as much as she talked to the contrary, she didn't want her best friend to get outed, and for her life to be made infinitely harder because of it.

"Dipper" Mabel gasped out, "fingers... in now... ohhhh" she trailed off, much too occupied by the sudden and intense entry of two of her brother's fingers into her soaked twat, as they pumped inside of her at a steady pace, driving her insane with the movements. He didn't just go in and out though, oh no. He'd learned from their many past trysts, and rotated his fingers, flexed and opened them, anything he could to make his invasion of her pussy trigger as many pleasurable nerves as possible.

And he succeeded too, as Mabel was breathing heavily on the table, head tucked into her arms to hide herself and her sounds from the room. Her friend just watched on, amused and aroused to see the Pines girl in such a state of intense delight, no doubt all the factors of her situation driving her up the wall, and not willing to pleasure herself for fear of giving herself away. So, she soldiered on, her pussy taking a strong pounding from her brother's fingers.

She squeaked lightly, the sound muffled by her arm, as he sped up, going faster and faster into her little snatch, thrusting and pounding at her with his fingers. She bit into her arm, moaning and squealing into her sleeve as her privates were being decimated by fingers from far away. Stacy looked around the room concernedly, but thankfully no-one had noticed her friend in the throes of pleasure. And on and on Mabel's pleasure built, going higher and higher, her cunt convulsing and gripping and clenching until...

It stopped.

Mabel popped her head out of her arms, confused, and still shuddering from the arousal, looked toward the front of the room where her brother sat. She saw him raise his hand, discreetly holding the portal in his pocket with the other. "Ms. Brown, may I be excused for the bathroom?" he said, voice completely normal, as if he hadn't just been pumping his fingers into his sibling's moments ago.

"Of course, Mr. Pines, just take the hall pass when you go." He nodded at the teacher's words and got up, walking down the aisle to the back of the room. His two accomplices in sexual risk watched him as he walked to the door, and as he passed right by their desk, she got a look at his face.

It was faint, and if you weren't attached to him at the hip since birth you might not catch it. But Mabel clearly saw the smallest smirk, one that spoke of perverse delights and a complete lack of mercy for his poor, mewling sister. Mabel blanched, torn between fear and arousal, the two conflicting emotions empowering each other and leaving the girl a flustered, sweating mess even before Dipper continued his long-distance fun.

But he was gone just as fast as he'd walked by, at a speed that could easily be seen as a need for the bathroom, but not be fast enough to draw undue attention.

When she saw the door shut, she swiftly turned to Stacy. "Stacy please, he's in the back!" she said in a whisper, not wanting to draw the entire class into her conversation, "You've gotta help me, I know this is my fault but I _seriously_ won't be able to stay quiet if Dipper goes to town on that!"

Hearing the quiet desperation in her friend's voice, Stacy shot up in her chair, drawing all eyes on her, and loudly proclaimed "Ms. Brown, I think Mabel's sick, can she go to the nurse's office?"

Mabel's head shot up, anxiousness and arousal both settling at the thought of Dipper starting with all these eyes on her. It was a close thing, but if they could leave the room soon enough, they might be able to make it to a closet or something before...

"Oh, is that true Ms. Fortwright?" Ms. Brown said, not a small amount of suspicion lacing her tone. "And isn't that the excuse you used just two weeks ago in order to put glitter bombs in the teacher's lounge?"

Stacy and Mabel both froze up at that, having entirely forgotten that incident, and cursing their lack of foresight. "Y-yes Ms. Brown, that's right." she said, slumping down in her seat.

Ms. Brown nodded at the girls. "Good, then you'll understand why I'm not jumping at the bit to trust that line from your mouth at this time." She then turned around, continuing her lesson and unknowingly making Mabel's situation much harder.

Mabel saw Stacy look at her and give a shaky, apologetic grin, but Mabel wasn't mad. She'd tried her best, but it was her own stupid glitter bombs and horny suggestions that had gotten her into this mess, it'd be her innate... Mabel-ness that would get her out! So she simply gave her friend a small thumbs up, and settled in to steel herself for her inevitable rough treatment at brother's hands.

She needn't have waited long, for within seconds she had to bite her lip at the familiar sensation of a tongue dragging itself across her folds, gathering up all her earlier fluids and revving up her engine like it'd never been turned off.

She felt the tongue slither around her snatch, moving and stretching her around like that's what it was born to do (which she _really_ liked to think it was), and clenched her fists to help stave off the noises she was inevitably going to start making soon. But she'd never been a particularly quiet lover in the first place, and when the tongue buried itself right in her love tunnel, she eeped into her teeth, biting down harder to prevent her classmates from finding anything amiss.

Mabel had always found her bro-bro to be very skilled with her mouth, and while she was _certainly_ enjoying that skill, it was truly testing her own skill of maintaining composure. She felt a hand grab her own and turned to see Stacy holding onto her, offering what little support she could in this scenario.

Mabel wasn't one to drop away from a perfectly good opportunity though, and grabbed her friend's hand, clenching it as tightly as she could as her brother's tongue explored the caverns of her pussy like it was made of gold.

Or like it was a wet vagina and he was a horny middle school boy, but really either one worked.

That train of thought was crushed when she felt his tongue wrap around her clit, squeezing it like a warm fleshy blanket, and Mabel was forced to bite into her arm to keep quiet. She breathed heavily into her arm as her brother's tongue carressed and massaged her love bud like it was his job. The attention made the already arduous task of keeping quiet even worse, and it was all she could do to ride the wave of intense pleasure her twin was almost assaulting her with. Shutting down her screams as much quieter throat noises, she bore the wave, feeling that previously stolen orgasm creeping up again on her, and oh _god_ she was so close, just a little bit closer...

And then it stopped, and Mabel almost screamed to the class in the sheer _frustration_ of being denied such sweet release twice in a row.

But that little bit of reason crept past the ocean of arousal in her brain, and held her back from doing so. She took a moment to breathe and recuperate while she had the chance. Short of running down to the bathrooms to clock her teasing, no-good brother, she knew she'd just have to ride out anything he was willing to do to her. After all, he had to come back to class sometime. His nerd sensibilities wouldn't let him stay away from his beloved learning for too long, right?

The feeling of his tongue probing her clenched anus wiped away all the willpower she had to hold out.

"Ms. B, please, I need to leave the room!" Mabel had given up all sense of secrecy at that feeling of prodding at her butthole. Dipper knew that was where she was weakest, he _knew that there was no chance in hell she'd be able to hold out against an anal attack at all._

__

Her teacher swiftly turned around, a very annoyed expression crossing her face. "Ms. Pines, please! Enough of these silly interruptions! I've given my word on this, so sit down and let me resume my lesson!" her teacher scolded, all the while herb legs shook under the sensation of her rosebud being thoroughly tasted.

__

Mabel had to persist though, because as much as the thought turned her on, she could _not_ be fucked into a drooling mess in the middle of math. Though, it would be really- no, focus! "Please Ms. Brown, I'm being s-ah! Serious here!" she pleaded, growing more desperate as her brother's muscle punched through her anal ring, and she knew she didn't have much time left.

__

She could see her teacher's face soften, the harsh lines of anger turning into creases of concern, if only slightly. "Are you absolutely certain Ms. Pines? If this is another trick, not only will I never let you out of this class again, you'll be serving in school suspension for a month!"

__

Seeing her chance, she pushed through the pleasure surging through her ass to respond. "Absolutely Ms. Brown! I _really_ need to go to the bathroom right n-AAAAA!" Mabel cut off, her brother's tongue swirling around her backdoor like a whirlpool, and her knees buckled, only supported by her arms on the desk and her friend's helpful hands on her stomach.

__

Her eyes closed, Mabel wasn't able to see her teacher's face, but she could hear the concern in her tone. "Okay then Ms. Pines, go ahead and get going. Ms. Fortwright, please escort Ms. Pines to the bathroom, and then the nurse's office, and don't make me regret this..."

__

Without a word, Mabel felt her friend lift her to her feet, and she began to make her way to the classroom door, the occasional moan and grunt forced out of her as she limped.

_When collapsed gripping the doorframe, Stacy there to help support her, it was all she could do to keep her cry of pleasure somewhat subdued, as the vigorous licking in her anus stopped and was swiftly replaced by the familiar presence of her twin's magnificent cock._

_She was on autopilot now, eyes closed and trusting her friend to guide her to... wherever as she focused on not alerting the whole school to the her predicament. Dipper had clearly been just as pent up as he'd made her, as he thrust inside her asshole like living depended on it. Mabel couldn't stop them entirely though, giving off a steady stream of moans and yelps at the relentless battering to her backdoor went on._

_Even on a normal day, her asshole was particularly sensitive, the combination of horny magical experimenting and a love of feeling full had lead to it becoming her most receptive hole, and she'd be crying out in pleasure in no time. But with the knowledge that she was just about as public and risky as possible right now, it was all she could do to keep her vocalizations of delight as quiet as they were, only the steady presence of her friend as well as the consequences of doing so kept her from screaming her throat raw._

_She weathered the unceasing pounding for what felt like forever, before she screamed in surprise at being forcefully sat down, opening her eyes to find herself seated on a toilet, Stacy's hands on her shoulders. "I've got the door to this bathroom locked and I checked all the stalls" her wonderful, amazing friend told her. "So go ahead Mabel. Go _nuts_."_

_And that she did, letting go of the little bit of control she'd retained to lose herself in the pleasure._

_" **OH FUCK YES DIPPER! I LOVE YOUR COCK SO MUCH! AAH!**_ " Her worries of being caught no longer an issue, Mabel screamed her enjoyment of a cock tunneling in her ass to the tile walls. It was heavenly, the risk and trepidation of being caught combined with the sensation of her brother's dick occupying every crevice of her young asshole made for the most enjoyable sensual experience to date.

Mabel's wild yells turned to raw pleasurable wailing as the feeling of a tongue in her vacant pussy joined the manic thrusting in her backdoor. She glanced down to see Stacy's mouth firmly planted onto her vulva, tongue tasting her twat like it was the single tastiest meal. She threw her head back, wailing and screeching her delight to the walls without a single care, the combined pleasure too much for her already frazzled brain to handle, and her legs stiffened and hands gripped the toilet seat intensely as a powerful orgasm rocked through her.

Her anus clenched tightly around the penis and her pussy around the tongue, forcing her two lovers to work harder to continue their mission, and causing Mabel's voice to rise another octave at the effort. Her body shook and her lady parts squirted what felt like an entire glass of juices into her friend's mouth, who gulped it all down without breaking stride. On her orgasm went, tearing through her for what seemed to be hours, until finally the penis in her ass thrust deeper and with more strength than before, and a wave of cum began filling her bowels.

Only a hoarse groan came from the girl's throat, so worn by her ragged pleasure, but it was just as intense as she felt her brother's hot semen pour into her poor poop-chute, filling up every nook and crevice. It kept going and going, a different previous horny experiment yielding a deluge of semen in her ass, until it was beginning to feel like she had two cocks fighting for dominance inside her backdoor. The cock pulsed and pulsed, each one bringing about more and more of that wonderful boy-sauce until finally, with an indignant sputter, it stopped.

Mabel collapsed upon that seat, harsh breaths coming through her as she sat and recovered from the fierce ordeal she was put through. A squeal came from her mouth, and she moved to push her friend away from her crotch. "G-gimme a sec Stacy, I... I need a breather..." she groaned. Her friend thankfully obliged, her face coming into Mabel's view and giving her a slow kiss, letting the Pines girl taste her own juices. There they sat, leisurely kissing for who knows how long, before the scraping of moving furniture alerted them both.

Mabel tried to get up but her overworked body barely responded, and a quick glance at her belly showed that the massive pool of semen in her ass was enough to hinder that further. Luckily for her, Stacy was quicker to get up and reached the door before it swung outwards, showing none other than Dipper Pines with a confident grin on his face.

"Hey Mabel, Stacy. You two look like you had _quite_ the time in here."

"Oh screw you bro" Mable drawled, "That w-was a close call with my a-ass there, and you know it."

He at least had the decency to look somewhat embarrassed. "Yeah, good point. Sorry sis, it won't happen ag-"

Mabel sprung to her feet, the force letting her fall onto the floor on top of her brother, as she tightly gripped his shirt. "Don't you **_dare_** tell me we're not doing that again!" she said, rage coloring her voice. "That was the best sex we've ever had, and we're absolutely doing that all the time from now on!"

Dipper gave a light chuckle. "Okay then, no problem, we'll do it again I promise. For now, let me just..."

Mabel felt his hands trying to reach under her, and she was suddenly aware that her brother's cock had never left it's spot. "Oh no you don't, there's enough cum inside my ass to fill a swimming pool! You caused this mess, you're gonna just have to stay snug in my tushie until we get home."

Dipper smiled brightly and opened his mouth, but before he could utter a word, he was cut off as a squeak emitted from Mabel, her shorts being pulled all the way down and a pair of hands spreading apart her buttcheeks. "Wow, guess that portal still lets you use her ass, even if it's in there. Say Mabes, you mind if I get some of that sweet, sweet bro-sauce?"

Her pussy heating up at the thought, she gave a thumbs up to her gal-pal. "Sure thing Stace, I want some too though so I guess we're gonna have Dippster here keep me topped off for the whole day..." she said lustily, before diving downwards and devouring her brother's lips.

As Stacy began to greedily devour her well-earned spunk deposit, Mabel felt her brother's cock start to shift again and his hips begin to slightly thrust. She moaned into his mouth, tongue entwining tongue. But before she delved entirely into depravity once more, she remembered that the teacher was probably expecting them back in math class soon, though she pushed the thought away as quickly as it came.

After all, books and learning had _nothing_ on her brother's cock and her girl-pal's tongue.


End file.
